1. Field of the Invention
This invention relates to an illumination optical system of an ophthalmologic instrument which includes a ring aperture plate for forming a ring-shaped image on a pupil of a subject's eye and a conical prism disposed between the ring aperture plate and an illumination light source.
2. Description of the Prior Art
Heretofore, there is known an illumination optical system of an ophthalmologic instrument shown in FIG. 6. In FIG. 6, reference numeral 1 designates an illumination optical system for illuminating a fundus Er of a subject's eye E and reference numeral 2 designates an observation optical system for observing the fundus Er. The illumination optical system 1 includes an objective lens 3, a perforated mirror 4, a relay lens 5, a mirror 6, a relay lens 7, a ring aperture plate or ring-perforated plate 8, a condenser lens 9, and an illumination light source 10. The ring aperture plate 8 has a ring-shaped aperture 8a. The illumination light source 10 is conjugate with the ring aperture plate 8 with respect to the condenser lens 9. The ring aperture plate 8 is conjugate with the perforated mirror 4 with respect to the relay lenses 7 and 5. The perforated mirror 4 is conjugate with a pupil Ea of the eye E with respect to the objective lens 3.
Illumination light emitted by the illumination light source 10 reaches the eye E via the condenser lens 9, ring-shaped aperture 8a, relay lens 7, mirror 6, relay lens 5, perforated mirror 4, and objective lens 3. A ring image of the ring-shaped aperture 8a is formed on the pupil Ea. The fundus Er is illuminated with the illumination light which has formed this ring image and has passed through a peripheral portion of the pupil Ea.
The illumination light, or rather observation light reflected by the fundus Er passes through a central portion of the pupil Ea and reaches the observation optical system 2 via the objective lens 3 and the perforated mirror 4 since the pupil Ea and the perforated mirror 4 are conjugate with each other. Accordingly, the fundus Er is observed.
As mentioned above, the illumination light reaches the eye E and passes through the peripheral portion of the pupil Ea to go to the fundus Er whereas the illumination light as the observation light is reflected by the fundus Er and passes through the central portion S of the pupil Ea (see FIG. 7) to go out of the eye E. For this reason, the illumination light and the observation light do not need to share a common optical path at an anterior portion of the eye E and therefore harmful light caused by reflection or scattering of the illumination light is removed at the anterior portion.
However, in the optical system of this type of conventional ophthalmologic instrument, the illumination light source 10, the ring aperture plate 8, the perforated mirror 4, and the pupil Ea are conjugate with each other. Accordingly, a filament image of the light source 10 is formed on the ring aperture plate 8.
Since only a part of the illumination light for forming the filament image, or only a part of the illumination light emitted by the illumination light source 10 passes through the ring-shaped aperture 8a, illumination efficiency is low. Accordingly, in order to raise the efficiency, an illumination optical system is proposed in which a conical prism Q shown by the broken line in FIG. 6 is disposed between the ring aperture plate 8 and the condenser lens 9 and thereby the illumination light emitted by the illumination light source 10 is changed into a ring-shaped beam of light to pass through the ring aperture plate 8a.
On the other hand, nowadays, in an ophthalmologic instrument for fluorescence photography, an optical system is proposed in which a laser source as the illumination light source is used. This optical system includes a laser source 11 and an optical fiber 12 for guiding a laser beam emitted by the laser source 11 which are each shown by the alternate long and short dash line in FIG. 6. An emission end 12a of the optical fiber 12 is located at the position of the illumination light source 10.
However, a core of the emission end 12a of the optical fiber 12 is extremely small in diameter. For example, it is 50 to 200 .mu.m. Therefore, if a laser beam emitted from the emission end 12a is changed into a ring-shaped beam of light by means of the conical prism Q, on the pupil Ea is formed a ring image of the ring-shaped beam S1 a width t of which is much narrower than a width T of the ring-shaped aperture 8a as shown in FIG. 8. Accordingly, a width t' of the ring image becomes extremely narrow as shown in FIG. 9.
Therefore, a problem arises where slight misalignment brings about a cut-off state (c-shaped, reversed-c-shaped, or crescent-shaped state) of the ring-shaped beam S1 because a part of the ring-shaped beam S1 is intercepted by an edge Eb of the pupil Ea and accordingly the fundus Er is illuminated unevenly.
That is, in the case of illumination light for forming a ring image R having the width T', the illumination light corresponding to an area shown by the slanted lines in FIG. 9 falls on the pupil Ea even if there is slight misalignment of the center Eo of the pupil Ea with respect to the center 0 of an optical axis of the observation optical system 2 in the up, down, right, and left directions. However, in the case of the ring-shaped beam S1 for forming a ring image having the width t', a part of the ring-shaped beam S1 is intercepted by the edge Eb of the pupil Ea (see the broken lines in FIG. 9) and, as a result, the fundus Er is not evenly illuminated with the ring-shaped beam S1.
It is a first object of the invention to provide an illumination optical system of an ophthalmologic instrument capable of avoiding the decrease of illumination efficiency without narrowing the width of a ring image formed on the pupil.
It is a second object of the invention to provide an illumination optical system of an ophthalmologic instrument capable of almost evenly illuminating the eye fundus regardless of slight misalignment.
It is a third object of the invention to provide an illumination optical system of an ophthalmologic instrument capable of forming a ring image on the pupil without narrowing the width of the ring image even in a case where a ring-shaped beam of light is made of illumination light emitted from an optical fiber small in diameter and falls on the fundus from the periphery of the pupil to illuminate the fundus.